The present invention relates to a process for the manufacture of aluminous cement from aluminum smelting residue admixed with other suitable materials and to the novel resulting cement.
The conventional aluminous cement has been manufactured in such a manner that bauxite or other suitable materials containing more than 50% Al.sub.2 O.sub.3 are mixed with limestone and the mixture is processed in an electric or reverberatory furnace. All proportions herein are by weight.
On the other hand, aluminum smelting residue as a by-product in the aluminum industry has usually been disposed of as waste material. However, since it contains some harmful chemical compounds such as aluminum nitride and aluminum carbide, which are liable to become a source of public hazards including air or water pollution, the proper disposal thereof has now become a matter of primary concern.
As a result of investigation of such residue, the present inventor has found that it is possible to manufacture aluminous cement therefrom and, further, that in the process of its manufacture, all of the aforementioned harmful chemical compounds contained in the residue are oxidized and, as a result, innocuous oxides and, in the case of aluminum nitride, nitrogen are generated. The reason for this is that the aluminum smelting residue contains as a main component thereof about 80% Al.sub.2 O.sub.3 in addition to aluminum nitride, aluminum carbide, zinc, tin, magnesium, vanadium, titanium, etc. and is essentially different in quality from bauxite as a natural raw material.
Referring to bauxite, there are a variety of kinds, but it may be roughly divided into two kinds, namely red and white. The former contains much ferric oxide, sometimes reaching as much as 20%, while the latter contains same as little as 2 to 8% ferric oxide, together with a comparatively small amount of SiO.sub.2, sometimes reaching as much as 20%. Since inclusion of SiO.sub.2 is to be avoided in smelting metallic aluminum, red bauxite is used as a basic material and ferric oxide contained therein is removed therefrom as a red slurry to thereby produce pure alumina for smelting to metallic aluminum. Accordingly, little SiO.sub.2 and iron are contained in the aluminum residue.
In this connection, reference is made to "the aluminous cement for refractory materials" of the Japan Industrial Standard R2511-1961, from which it will be seen that the allowable content of Al.sub.2 O.sub.3 in the first class of such cement is more than 50% and more than 35% in the second class, Fe.sub.2 O.sub.3 is below 3% in the first class and below 10% in the second class and CaO is below 40% in both classes but no standards are given regarding SiO.sub.2 content. However, as a result of investigation by the present inventor, it has been found that inclusion of up to about 15% SiO.sub.2 and/or up to about 15% Fe.sub.2 O.sub.3 in aluminous cement is permissible and that these ingredients in proportions below 15% can improve the efficiency of the cement.
On the other hand, as a matter of practice, the conventional aluminous cement contains little SiO.sub.2 or Fe.sub.2 O.sub.3 since it is manufactured by an electrical fusing method in which a state of oxidation is brought about such that ferric oxide and SiO.sub.2 contained in the batch are reacted to produce ferrosilicon, which is then separated from the components of the cement. Likewise, in the case of the recently developed reverberatory furnace being employed, with which a fusing method is effected, a state of natural reduction is brought about and iron and silica are combined to generate ferrosilicate, which tends to separate from the cement.